General Characteristics of Phylum Echinodermata

(Echinos: Spines; derma: Skin)

Phylum Echinodermata forms the highly advanced group among invertebrates.The examples of this group are star-fishes, sea-urchins,sea-cucucmbers etc.This phylum is charecterised by the following salient features,

1. Habitat and Habit: Echinoderma are exclusively marine. They generally live at sea bottom borne are pelagic (free swimming in open water) and a few are sessile (attached to the substratum).

2. They are Multicellular organisms with organ Grade of organisation

3. Blasticity: They are Triploblastic animals

4. Coelom: They are coelomate animals

5. Symmetry: The symmetry is bilateral in larvae but pentamerous radial in adults i. e., body parts are arranged in fives or multiples of five.

6. They have well developed endoskeleton formed of spines and pincer-like pedicellariae. The spines are protective in function. The pedicellariae keep the body surface clear of debris and minute organisms.

7. Ambulacral System (= Water Vascular System): Presence of ambulacral system is the characteristic feature of phylum echinodermata. A perforated plate called madreporite is present in this system. The pores of the madreporite allow water into the system Tube teet of this system help in locomotion, capture of food and respiration. Water vascular system is of coelomic origin. 8. Circulatory system is open type and includes haemal and perihaemal systems. The so called blood is often without a respiratory pigment. There is no heart. 9. Respiratory Organs: Gaseous exchange occurs by dermal branchae or papulae in star fishes peristominal gills in sea urchins, genital bursae in brittle stars, and cloac respiratory ‘trees in holothnrians. Exchange of gases also takes place through tnbe feet.

10. Excretory Organs: Specialized excretory organs are absent. Nitrogenous wastes are diffused out via gills. Ammonia is chief excretory matter.

11. Nervous System: It consists of a nerve ring and radial nerve cords. Brain as such is absent.

12. Sense Organs: They are poorly developed.

13. Sexes and Fertilization: Except a few individuals, the sexes are separate. There is no sexual dimorphism. Fertilization is usually external.

15. Asexual Reproduction: Some forms reproduce asexually by self-division.

16. Autotomy and Regeneration: Phenomena of autotomy and regeneration are often well marked in echinoderms.

17. Development:

The development is indirect and includes a ciliated, bilaterally symmetrical larva that undergoes metamorphosis to change into the radially symmetrical adult. Water vascular system videos

https://www.youtube.com/watch?v=fcILzBZ_Aj4 https://www.youtube.com/watch?v=K2G7L5hcEt8

CLASSIFICATION OF PHYLUM ECHINODERMATA Phylum Echinodermata is divided in to 2 subphylum 1. Subphylum Eleutherozoa: Three classes-Asteroidea, Ophiuroidea, Echinoidea 2. Subphylum Pelmatozoa: One class with living organisms, Crinoidea

Sub phylum 1. Eleutherozoa 2. Pelmatozoa Class Asteroidea Ophiuroidea Echinoidea Holothuroidea Crinoidea Sea star brittle stars and (Gk. echinos- hedgehog, Sea cucumber Sea lilly or sea feather (Gk. aster- star, eidos- basket stars (Gk. eidos- form) (Gk. Holothurion- (Gk. Crinon- lily, eidos- 1. Common form) Ophis - snake, sea cucumber, form) name Oura- tail, eidos- form): eidos- form) Body is flattened star Body is flat with Body is disc-like or oval Body shape varies, Body is star shaped shaped with five arms pentamerous disc or heart shaped may be elongated 2. Body which are not sharply like cucumber or shape marked off from the rounded or five central disc sided Presence of They possess a They possess compact They have no arms, Arms are branched. Spines calcareous plates and long arm which is skeleton and movable spines and and pedicellariae and 3. movable spines. sharply demarcated spines. No free arms pedicellariae. madreporite are absent Spines/arms from the central disc

Oral and aboral Oral and aboral Mouth on the oral and Mouth and anus at Mouth is present in the surfaces are quite surfaces are quite anus on the aboral the opposite ends centre and food grooves distinct. Mouth on the distinct. Anus is surface. Biting and radiate along the oral 4. Mouth oral and anus on the wanting chewing apparatus with surface of arms aboral surface teeth called Aristotle’s Lantern is present

Class Asteroidea Ophiuroidea Echinoidea Holothuroidea Crinoidea Ambulacral grooves are Ambulacral grooves are Ambulacral grooves are The water vascular Crinoids have no on the oral surface. absent. The madreporite absent. The madreporite system has no madreporite. Instead, Water enters the system is located on the located within one of the connection to the the oral surface is through a sieve-like underside of the animal, plates surrounding the anus outside, and is thus dotted within 5.Water structure on the upper usually in one of the jaw on the upper surface of the filled with the numerous vascular system surface of the animal, plates animal. The stone canal internal coelomic fluid, minute ciliated funnels called the madreporite. descends from the rather than sea water. that run into the main madreporite to the ring The madreporite is body cavity canal present

They possess tube feet They possess tube feet They possess tube feet The tube feet are They have tube feet with a sucker without sucker with a sucker. sucking type which is without suckers 6.Tube feet modified into tentacles and form a circle around mouth

Larval forms are Larval form is Larval forms are Platens Larval forms are Larval form is 7. Larva Bipinnaria and Ophiopluteus and Echinopluteus Auricularia Doliolaria Brachiolaria Asterias (Star fish), Ophiothrix (brittle star), Echinus (Sea Cucumaria (Sea Antedon (feather star). Astropecten, Ophioderma (brittle star) urchin), Cidaris, cucumber), Holothuria, Sea lilies

8. Examples

SEA STAR Phylum: Echinodermata Class: Asteroidea Genus: Asterias Asterias is commonly known by the name of starfish. The name starfish is somewhat misleading suggesting an organism to be like a star and fish. Therefore, recently it is renamed as sea star. Habit and Habitat of Asterias:

Asterias is exclusively marine, bottom dwelling or benthonic animal, inhabiting in the bottom of sea. Most of them are nocturnal, remain quiet in day time and become active during night. They move by crawling on the bottom, mostly at a rather slow rate. All sea stars are carnivorous and feed voraciously on almost any available slow moving or sessile animals, chiefly on polychaetes, crustaceans, molluscs and other echinoderms. External Features of Asterias: (i) Shape, Size and Colour: Asterias has a radially symmetrical and pentamerous body. The body consists of a central, pentagonal central disc from which radiate out five elongated, tapering, symmetrical spaced projections, the rays or arms.The size varies from 10-20 cm in diameter though some forms may be much smaller or longer. The colour is variable having shades of yellow, orange, brown and purple.

The body has two surfaces, the upper convex and much darker side is called the aboral or abactinal surface. The lower surface is flat, less pigmented and is called the oral or actinal surface.

(ii) Oral Surface: The lower of body, which remains towards the substratum and contains the mouth or oral opening, is flat and of dark orange to purplish colour, is oral actinal surface. The oral surface bears the following structures:

1. Mouth: On the oral surface, in the centre of the pentagonal central disc is an aperture, the actinosome or mouth. Mouth is with five angles, each directed towards an arm. The mouth is surrounded by a soft and delicate membrane called peristome and is guarded by five groups of oral spines or mouth papillae.

2. Ambulacral Grooves: From each angle of the mouth radiates a narrow groove called the ambulacral groove which runs all along the middle of oral surface of each arm.

3. Tube Feet or Podia: Each ambulacral groove contains four rows of locomotory, food capturing, respiratory and sensory organs called tube feet or podia. The tube feet are soft, thin- walled, tubular, retractile structures provided with terminal discs or suckers. The suckers function as suction cups to afford a firm attachment on the surface to which they are applied.

4. Ambulacral Spines: Each ambulacral groove is bordered and guarded laterally by 2 or 3 rows of movable calcareous ambulacral spines which are capable of closing over the groove. Near the mouth, these spines often become larger, stouter, assemble in five groups, one at each inter-radius of disc and are called mouth papilla. Outside the ambulacral spines are three rows of stout immovable spines, beyond which occurs another series of marginal spines along the borders of the arms demarcating the oral from the aboral surface.

5. Sense Organs: Sense organs include five unpaired terminal tentacles and five unpaired eye spots. The tip of each arm bears a small median, non-retractile and hollow projection, the terminal tentacle. It acts as a tactile and olfactory organ. At the base of each tentacle occurs a bright red photo-sensitive eye spot made up of several ocelli. (iii) Aboral Surface: The upper convex surface is called aboral or abactinal surface.

The aboral surface bears following structures:

1. Anus: A minute circular aperture, called the anus, is situated close to the centre of the central disc of aboral surface. 2. Madreporite: At the aboral surface of the central disc occurs a flat, sub-circular, asymmetrical and grooved plate called madreporite plate or madreporite between the bases of two of the five arms. The surface of madreporite is marked by a number of radiating, narrow, straight or slightly wavy grooves with pores in them. The madreporite is a sieve-like porous plate and it leads to the stone canal of water vascular system. The two arms having madreporite between their bases are collectively referred to as a bivium and the other three arms as a trivium. The symmetrical position of madreporite, thus, converts the radial symmetry of Asterias into bilateral symmetry.

3. Spines: The entire aboral surface is covered with numerous short, stout, blunt, calcareous spines or tubercles arranged in irregular rows running parallel to the long axes of the arms. The spines are supported by the irregularly-shaped calcareous plates or ossicles which remain buried in the integument and form the endoskeleton.

4. Papulae or Gills: Between the ossicles of integument are present a large number of minute dermal pores. Through each dermal pore projects out a very small, delicate, tubular or conical, finger-like or thread-like, thin-walled, membranous and retractile projection called the dermal branchia or gill or papula. The papulae are hollow evaginations of the body wall and their lumen remains in continuation with the coelom. They are internally lined by coelom. They have respiratory, as well as excretory functions.

5. Pedicellariae: Pedicellariae are microscopic, pincer like organs found all over the body on both oral and aboral surfaces.Theses are found between spines.These are modified spines. Types of Pedicellaria The Pedicellaria of star fish are provided with three ossicles. Hence they are called forcipulate. These Pedicellaria are of four types. They are of four types, (i) Straight type: In this type the jaws are straight and they remain parellell throughout their length like forceps. (ii) Crossed type: In this type the basal ends of the two jaws are curved and they cross each other.They work like scissors iii) Stalked Pedicellaria: This type has stalk iv) Sessile Pedicellaria: This type has no stalk Functions of Pedicellariae: Organs of offense and defence, clean the body surface, prey capture, protect papulae.

DIGESTIVE SYSTEM OF ASTERIAS The digestive system of Asterias extends vertically from oral to aboral side in the central disc. The alimentary canal is tubular, straight and much shortened by the flattening of the body. It is formed of Alimentary Canal and digestive glands.

Alimentary Canal: Comprises the following parts: 1. Mouth: Oral surface bears the mouth situated in the centre of the disc. It is surrounded by peristomial membrane. The mouth leads upward into the oesophagus. 2. Oesophagus: The oesophagus is a very short, wide and vertical tube. It opens aboraly in the stomach. 3. Stomach: The stomach is a broad sac and fills the interior of the disc. Stomac consist of two parts part, cardiac stomach immediately following the oesophagus and a pyloric stomach lying above it. Cardiac stomach is a five lobed sac. It is capable of being everted outside while the animal feeding. The retraction of cardiac stomach is brought about by five pairs of retractor muscles which arise from the lateral sides of the ambulacral ridge. The pyloric stomach is much smaller, flat and pentagonal sac. 4. Intestine: Pyloric stomach communicates with the intestine. The intestine is a short, narrow, five sided tube that runs straight upward to open out at the anus. It gives off two or three little hollow diverticula called intestinal or rectal caeca placed inter-radially. 5. Anus: The intestine opens on the aboral surface by a posterior most aperture of the alimentary canal called anus. The anus is situated eccentrically on the aboral side of central disc

Digestive Glands: The digestive gland of Asterias is called pyloric caeca. There are five pairs of pyloric caeca, one pair in each arm. Each pyloric caecum is formed of two rows of glandular pouches. They open into a median duct. The two ducts of an arm unite together to form a common duct called pyloric duct. The pyloric duct opens into the pyloric stomach.The pyloric caeca are suspended from the roof of the arms by paired mesenteries. The pyloric caecum is formed of four types of cells namely, ciliated cells, enzyme producing cells, mucous cells and storage cells.

Feeding and Digestion: Starfish is carnivorous and it feeds on crustaceans, molluscs and fishes. When a starfish comes across a mussel, it bends its flexible arms over the body of the mussel and attaches the tube feet to the two valves with their sucks. When the two valves are separated, the cardiac stomach is everted out and inserted between the valves and covered over the body of the mussel. Then the mussel is digested by the digestive juices poured over it. When the digestion is completed the cardiac stomach is withdrawn into body. Thus digestion is extracellular

The enzymes digest the visceral organs of the mussel (prey/victim), they; thus, convert the proteins of visceral organs into peptones and amino acids; polysaccharide carbohydrates and lipid contents into fatty acids and glycerol. When the digestion is partially completed the sea star withdraws its stomach along with the digested food by means of its retractor muscles and moves on leaving behind the shell of the victim.

Remaining digestion of food substances occurs in cardiac stomach (ie, intracellular digestion) from which the digested food diffuses to coelomic fluid for body wide distribution and to pyloric caeca where it may be distributed to arms or stored in storage cells of epithelium of pyloric caeca. The undigested foods is either egested out directly from the mouth or pass through intestine or egested out through the anus.

WATER VASCULAR SYSTEM OF ASTERIAS: The water vascular system is a modified part of coelom and it consists of a system of sea- water filled canals having certain corpuscles. It plays most vital role in the locomotion of the animal and comprises madreporite, stone canal, ring canal, radial canal, Tiedeman’s bodies, lateral canals, and tube feet

(i) Madreporite: Madreporite is a rounded calcareous plate occurring on the aboral surface of the central disc in inter-radial position. Its surface bears a number of radiating, narrow, straight or wavy grooves or furrows. Each furrow contains many minute pores at its bottom.

Each pore leads into a very short, fine, tubular pore canal which passes inward in the substance of the madreporite. There may be about 200 pores and pore- canals. The pore-canals unite to form the collecting canals which open into an ampulla beneath the madreporite.

(ii) Stone Canal: The ampulla opens into a S-shaped stone canal. The stone canal extends downwards (orally) and opens into a ring canal, around the mouth. The walls of stone canal are supported by a series of calcareous rings. The lumen of stone canal is lined by very tall flagellated cells.

In embryonic stages and in young Asterias, the stone canal remains a simple tube but in adult Asterias, lumen of stone canal possesses a prominent ridge with two spirally rolled lamellae which by branching become more complicated in structure. During its course, the stone canal is en-sheathed by a wide, thin-walled tubular coelomic sac, called axial sinus.

(iii) Ring Canal: The ring canal or water ring is located to the inner side of the peristomial ring of ossicles and directly above (aboral) to the hypo neural ring sinus. It is wide and pentagonal or five sided.

(iv) Tiedemann’s Bodies: The ring canal gives out inter-radially nine small, yellowish, irregular or rounded glandular bodies called racemose or Tiedemann’s bodies, from its inner margins. The Tiedemann’s bodies rest upon the peristomial ring of ossicles. The actual function of Tiedemann’s bodies is still unknown, however, they are supposed to be lymphatic glands to manufacture the amoebocytes of the water vascular system.

(v) Polian Vesicles: The ring canal gives off on its inner side in the inter-radial position one, two or four, little, pear- shaped, thin-walled, contractile bladders or reservoirs with long necks called polian vesicles. They are supposed to regulate pressure inside ambulacral system and to manufacture amoeboid cells of ambulacral system.

(vi) Radial Canal: From its outer surface the ring canal gives off a radial water canal into each arm that runs throughout the length of the arm and terminates as the lumen of terminal tentacle. In the arm the radial water canal runs immediately to the oral side of the ambulacral muscles.

(vii) Lateral Canals: In each arm, the radial canal gives out two series of short, narrow, transverse branches called lateral or podial canals. Each lateral canal is attached to the base of a tube foot and is provided with a valve to prevent backward flow of fluid into the radial canal.

(viii) Tube Feet: As already mentioned, there are four rows of tube feet in each ambulacral groove. A tube foot or podium is a hollow, elastic, thin-walled, closed cylinder or sac-like structure having an upper sac-like ampulla, a middle tubular podium and a lower disc-like sucker..

REPRODUCTIVE SYSTEM OF ASTERIAS

Most species of Asterias are unisexual or dioecious, i.e., sexes are separate except a few species such as Asterias rubens which is hermaphrodite. There is no marked sexual dimorphism, however, during breeding season some sort of colour difference between both the sexes may occur. The reproductive organs of Asterias are of primitive type and lack copulatory organs, accessory glands, receptacles for storing ova and reservoirs for storing mature sperms. There are only gonads which act as reproductive organs.

Gonads: The male gonads are testes and female gonads are ovaries. Each sexually mature male or female individual contains five pairs of testes or ovaries; one pair is lying free laterally in the proximal part of each arm between the pyloric caeca and the ampullae. The testes and ovaries are morphologically similar. Each gonad appears as an elongated feathery tuft or tuft of tubules or bunch of grapes, whose size varies greatly according to the proximity of spawning time.

At maturity the gonads occupy the entire perivisceral space. The proximal end of each gonad is attached to aboral body wall near the inter-brachial septum by a very short gonoduct which is ciliated and opens laterally through a small gonopore on the aboral surface almost at the angle of two adjacent arms. Each gonad is enclosed in a genital sac of coelomic nature with a wall of muscle and connective tissue fibres, covered externally with peritoneum. This genital sac is the outgrowth of the genital or aboral coelomic sinus. The gonad proper is lined by germinal epithelium, containing the germ cells. The mature sperms and ova are discharged by male and female Asterias respectively in sea water. The release of sex cells from the gonads is regulated by neuro-hormonal secretion of radial nerve.

6. The mouth represents the oral side and anus represents the aboral side 7. Five lobes grow between the mouth and the anus. There are 5 arm rudiments 8. The bilateral symmetry is transformed into radial symmetry. Autotomy of Asterias: Asterias possesses considerable power of regeneration. It is capable to regenerate any lost part of its body at any time. Moreover, if an arm is injured or held up, Asterias usually casts it off near the base at the fourth or fifth ambulacral ossicle. This is called autotomy. The broken off arm is immediately closed by the contraction of the adjacent body wall musculature for the protection of internal body organs and regeneration of new arm starts at that place. A disc deprived of all its arms regenerates. In Asterina vulgaris, a single arm with a portion of disc regenerates an entire animal. But in Linckia, an arm totally devoid of disc can also regenerate complete animal (Fig. 85.15).

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